1. Field of the Invention:
Spectacle and eyeglass lenses with particular reference to an improved general purpose lens series employing an aspheric, atoric design concept.
2. Description of the Prior Art:
The ophthalmic lens art includes much theory concerning correction for astigmatism with and without consideration of power change and the provision of lenses in multibase series. U.S. Pat. Nos. 989,645; 1,315,667; 1,438,820 and 1,438,827 are exemplary of early design work.
More comprehensive lens design work was disclosed by Tillyer in U.S. Pat. No. 1,588,559 which illustrates how lenses are computed.
The idea that the distance to the center of the eye which enters into the calculations, should be varied according to the power of the lens was introduced by Hill and Tillyer, U.S. Pat. No. 1,315,667 and Tillyer U.S. Pat. No. 2,391,045 which were both involved with compromising the correction between astigmatism and power change.
Davis et al U.S. Pat. Nos. 3,434,781 and 3,169,247, which respectively deal with matters of a negative toric lens series and an aspheric lens series for aphakic patients, delved still further into matters of ray tracing in lens design work and correction or minimization of errors of oblique fields while taking into consideration the various different physiological dimensions of different eyes.
Traditional design objectives in terms of tangential and sagittal meridional power errors for lenses have placed a priority on maintaining these errors below visually significant values leaving little freedom in choice of base curve, even for a single center of rotation distance.
The fundamental philosophy of designers having heretofor been to maintain tangential and sagittal meridional power errors within reasonable limits as a dominant consideration has resulted in their having to ignore other lens performance characteristics. Distortion, reflections, cosmetic appearance, magnification and magnification differences between the two eyes were secondary considerations and exemplary of those usually having to be ignored. The limitation on choice of curvature is even more acute if a range of center-of-rotation distances is considered rather than a single one.
From a study of aspherics (aspheric surfaces) it has been found that their use heretofor in the normal non-aphakic prescription range has not resulted in a major improvement of the traditional corrections. In these ranges of conventional design practices, aspheric surfaces can, at best, provide only slight improvement of meridional power errors.
In view of the fact that aspheric surfaces do offer the freedom to obtain one's "best choice" within a wide range of base curve selections, a principal objective of this invention is to apply the base curve as a new variable which may be used to achieve other design goals. Exemplary of these are the following lens performance characteristics which are a function of base curve and base curve changes:
a. Shape magnification and shape magnification differences between adjacent base curves PA1 b. Sensitivity of field errors to fitting distances PA1 c. Geometric distortion PA1 d. Applicability to large spectacle frames and cosmetic appearance in general PA1 e. Eyelash clearance PA1 f. Reduction of reflections